Serum or plasma concentration of vitamin B12 (cobalamin)

A vitamin B12 blood test measures how much of this essential vitamin is in your body. Your doctor may order it if you have ongoing fatigue, weakness, numbness or tingling in your hands or feet, balance problems, memory trouble, or certain types of anemia, or if your diet may not provide enough B12. The results help find the cause of your symptoms and guide the right treatment plan for you.

Your doctor may order this test to check for vitamin B12 deficiency, which can cause fatigue, numbness, or weakness. It may also be used to evaluate conditions like pernicious anemia, certain gastrointestinal disorders, or malabsorption issues.

Preparation for this test is usually very simple. Tell your doctor about every medicine, vitamin, and supplement you take, because certain drugs, such as metformin or stomach acid reducers, can affect your B12 levels. Fasting is often not required for this test alone, but if it is combined with other blood work, you may be asked not to eat or drink for several hours beforehand. Wear a sleeve that is easy to roll up.

No special preparation is needed, but you may be asked to fast for 8–12 hours before the draw if your lab uses a different protocol. Bring any medications you’re taking to your appointment.

During the test, a trained healthcare professional will wrap a soft band around your upper arm, clean your skin, and draw a small sample of blood from a vein, usually on the inside of your elbow. You may feel a quick pinch or sting. The collection takes only a few minutes, and afterward a cotton pad and small bandage are placed on the site. You can return to your normal activities right away.

A healthcare provider will draw blood from a vein in your arm. The sample is sent to a lab for analysis, where vitamin B12 levels are measured using an immunoassay method.

Results are reported as a number compared with the laboratory's reference range, which can vary slightly from one lab to another. Low levels may point to a diet lacking B12, pernicious anemia, or conditions that prevent your body from absorbing the vitamin, while high levels are uncommon but can occur with liver disease or certain treatments. Your doctor will explain what your specific result means and whether further testing is needed.

Results are typically reported as a concentration in units per liter (e.g., pg/mL or pmol/L). Low levels may indicate deficiency, while high levels could signal conditions like certain cancers or liver disease.